Update - This issue has been fully resolved.
Oct 12, 09:48 PDT

Update - This will not prevent any meetings from being scheduled. To add Highfive meetings, affected users can simply sign in for each calendar event they create and everything after that will work correctly.

However, clearly no one should have to sign in each time a meeting is created, so this is our highest priority.
Oct 6, 16:01 PDT

Update - This appears to only be happening in the Chrome browser and is affecting all users (previously reported as only some).
Oct 6, 15:56 PDT

Investigating - When creating a O365 meeting event in Outlook and adding a Highfive meeting, users are unexpectedly being asked to log in every time.

This only occurs when using the Highfive add-in (this is the one that opens a sidebar fly-out), and not with the VSTO plug-in (which is used for older versions of Outlook).
Oct 6, 15:55 PDT